This invention relates in general to wireless telecommunications networks and applications and, in particular, to a method and system of identifying and analyzing the source of uplink/downlink interference in a disturbed cell of a wireless telecommunications network. More particularly, the invention relates to methods of identifying sources of interference that can cause a device to seal or voice quality to degrade, making it unavailable for use in the network.
Without limiting the scope of the invention, its background is described in connection with identifying and analyzing downlink interference in a wireless telecommunications network, as an example.
Present-day mobile telephony has spurred rapid technological advances in both wireless and wireline communications. The wireless industry, in particular, is a rapidly growing industry, with advances, improvements, and technological breakthroughs occurring on an almost daily basis. Many mobile or wireless telecommunications systems, among them the European GSM-system, have passed through several generations of advancements and development phases, and system designers are now concentrating on further improvements to such systems, including system refinements and the introduction of optional subscriber services.
Most wireless telecommunication systems are implemented as cellular telephone networks wherein a group of Base Transceiver Stations (BTS), or base stations are served by a centrally located switch. The switch is commonly referred to as a Mobile Switching Center (MSC). The base stations are spaced apart from each other by distances of between one-half and twenty kilometers. Each base station is assigned a number of two-way voice and control channels. The voice channels transmit voice signals to and from proximately located mobile stations, and transmit control information to and from these mobile stations, usually for the purpose of establishing a voice communications link.
A typical cellular telephone network also includes a group of interconnected MSCs, which operate in association with a gateway mobile switching centerthrough which the cellular telephone network interconnects with a conventional Public Switched Telephone Network (PSTN). In addition, at least one Home Location Register (HLR) operates within the cellular telephone network. The HLR stores network subscriber information, including the identified location of current mobile stations within the network.
In response to an incoming call placed to a mobile station, the MSC queries the HLR to determine the current location of the station. The HLR xe2x80x9clooks upxe2x80x9d the current location of the mobile station and contacts the currently serving MSC to pre-route the call and retrieve a temporary location directory number, which is utilized to route the call through the telecommunications network for termination of the call to the mobile station. The MSC instructs the base station serving the cell in which the mobile station is located to page the mobile station. Responding to the page, the mobile station requests assignment of a channel, and the network terminates the call through the serving MSC and over the assigned channel.
Calls by mobile subscribers can be affected by interference which can cause radio disturbance events which, in turn, limit the efficiency of the network. As such, it is important to identify those cells within the network, which are sources of and subject to radio disturbance events. Interference itself can be either external or internal to radio network. The internal interference results from call activities within a network cell site. It is appropriate to term of cells as either xe2x80x9coffendingxe2x80x9d or xe2x80x9cdisturbed.xe2x80x9d A radio disturbance event typically occurs during a cellular call, either on the downlink (from a base station to a mobile station) or on the uplink (from a mobile station to a base station). The mobile phone is also called a xe2x80x9cportablexe2x80x9d or xe2x80x9ccellular phonexe2x80x9d. The disturbance event can be limited to several types of interference, including co-channel interference, adjacent channel interference or external interference.
Various methods exist for determining when a cell has been disturbed. Typically, a comparison of signal strength versus a measurement of speech quality can be employed to determine the Bit Error Rate (BER) of the transmission channel. When sufficient signal strength is correlated with degraded speech quality for an extended period of time (usually measured in seconds) that cell can be considered xe2x80x9cdisturbed.xe2x80x9d Failure to identify and analyze sources of such disturbances could result in poor channel quality and the sealing of devices which means they are unavailable for use in handling calls.
Various methods and systems currently exist for identifying disturbed cells within the wireless telecommunication networks. One of the most widely utilized methods involves downlink interference prediction tools, or prediction methods which use model-based prediction algorithms. Such tools predict where interference may exist within a given network coverage area. The predictions are then utilized for frequency and cell planning, particularly in initial network designs. The validity of such predictions is dependent on a number of factors, including the accuracy of the propagation model utilized, the resolution of the terrain data, and so forth. Such tools are helpful in identifying the cells that are causing downlink interference, but taken together are often inaccurate because of the dependence on predictions. That is, such prediction tools do not always account for xe2x80x9creal-lifexe2x80x9d sources of interferences in the coverage area as determined through more empirical measurement methods.
Another method utilized to identify disturbed and offending cells involves drive testing by field personnel. The drive test can be performed by turning off all adjacent/co-channel transmitters for a particular disturbed cell and then keying up each transmitter individually. A drive test team, in the meantime, drives the area in a motorized vehicle to observe and measure interference within the drive area. This method is inherently labor intensive and costly. In addition, the drive-test approach, while sometimes useful, does not take into account variations in mobile station types and can be quite expensive since it requires continuous measurement by field personnel. It also limits the possible offenders that can be investigated.
In short, the prior art methods, or approaches, of analyzing the interference in the network coverage area are generally unsuitable for today""s modern wireless network. What is needed is a cost effective method of identifying and analyzing sources of interference in the network that utilizes available qualitative data about the network. A means of identifying sources of interference, or offending cells, along with the disturbed cells in the network would provide numerous advantages.
The present invention provides a method and system for identifying and analyzing sources of uplink interference in a telecommunications network. With the present invention, the network operator can identify sources of interference and use this information in designing the network or improving performance.
Disclosed in one embodiment is a method of identifying the source of interference in a telecommunications network including one disturbed cell and a plurality of offending cells. The method comprises the steps of recording call events occurring in the offending cells as well as disturbance events occurring in the disturbed cell of the telecommunications network. The recording steps are run in conjunction with each other for the purpose of correlating and computing the two events.
The method also comprises the step of correlating recorded call events with recorded disturbance events as a function of time. The time stamps of the call start attempts (voice channel seizure events) result in Call Event Recordings (CER) for possible adjacent channel/co-channel mobiles. The correlating step is thus followed by a counting step, which comprises identifying the number of disturbance events associated with the disturbed cell. Once a call event is initiated, if sufficient interference is created, a disturbance event in the disturbed cell will be recorded. During the duration of the call event, the disturbance event will remain present. As a result, once the call event ends, the disturbance event will end within an time period delta t (xcex94t), therefore triggering the counter step.
The method further comprises the step of computing a distribution of disturbed and offending cells within the telecommunications network as a function of time to obtain a statistical correlation of call events in offending cells and subsequent radio disturbance events resulting in the disturbed cell. This step is performed to identify the possible sources of disturbances within the disturbed cell.
The method also comprises the step of verifying disturbance events. This verification step is performed using free space path loss considerations. All co-channel cells for which calculated signal attenuation indicates they are too far away to generate perceived disturbance events are excluded from further analysis. The propagation model takes into consideration the geographical position, antenna direction, the antenna height, the Effective Radiated Power (ERP), and so forth.
The internal sources of interference on the downlink are also identified using reciprocity. The concept of reciprocity dictates that if the mobiles of the offending cells create interference on the uplink for the disturbed cell, then by the rule of reciprocity, the disturbed cell may disturb the mobiles of the offending cells on the downlink. Therefore, the cells that are disturbed on the uplink are potential candidates to consider as those who disturb their offending cells, on the downlink. Hereinafter, xe2x80x9coffenderxe2x80x9d and xe2x80x9coffending cellsxe2x80x9d may be used interchangeably. Also, xe2x80x9cinterfered cellxe2x80x9d and xe2x80x9cdisturbed cellxe2x80x9d may be substituted for one another.
Technical advantages of the present invention include a less labor intensive method of identifying disturbed cells in the network compared to the drive testing approach. Sources of interference are identified in the shortest amount of time and by use of disturbance data already contained in the network.
Other technical advantages include more accurate identification and analysis of interference sources. The method and system of the present invention utilize empirical measurements based on recorded disturbance events and not predictions. As such, the interference analysis takes into account the behavior of all mobiles within the network rather than a particular mobile event.